The Fischer-Tropsch (hereinafter, abbreviated as “FT”) synthesis is a technique developed by German chemists Franz Fischer and Hans Tropsch in 1920, which produces synthetic fuel (i.e., hydrocarbons) from syngas (i.e., hydrogen and carbon monoxide) through the following reaction.(2n+1)H2+nCO→CnH(2n+2)+nH2O  [Reaction]
Catalysts containing cobalt and iron are primarily used in such Fischer-Tropsch synthesis, and reaction conditions, such as reaction temperature, reaction pressure, gas composition, and the like, are determined based on the type of catalysts used.
The Fischer-Tropsch synthesis, according to reaction temperature, can be largely classified into low-temperature Fischer-Tropsch synthesis (LTFT), which is carried out at a temperature range between 200° C. to 250° C., and high-temperature Fischer-Tropsch synthesis (HTFT), which is carried out at a temperature range between 300° C. to 350° C. (Andrei Y. Khodakov et al, Chem. Rev., 2007, 107, 1672).
Traditionally, iron-based catalysts are used in the HTFT synthesis. Since the iron-based catalysts also show activity to the water-gas shift reaction, they can be used as various compositions in which a syngas ratio of hydrogen and carbon monoxide varies between 1 and 2, and further used in the presence of carbon dioxide, a gas impurity.
Further, the iron-based catalysts have been applied in commercial FT processes such as CTL (coal-to-liquid) as they are inexpensive and are strongly tolerant to sulfur-containing compounds. A representative example of the commercial processes includes the synthol process, which uses iron catalysts made of fused iron produced from Sasol Limited.
Recently, research demonstrating the successful application of a highly active catalyst having carbon bodies as a supporter for the iron catalysts in the high-temperature FT synthesis have been reported (Krijin P. de Jong. et al. Science, 2012, 335, 835). For example, in the case of materials composed of carbon, such as carbon nanotubes (CNT), carbon nanofibers (CNF), activated carbon, and the like, they are stable towards steam generated during the high-temperature FT synthesis and are advantageous in heat transmission, and the inside of the carbon supporter can provide much more favorable conditions for reduction and activation of particles. Therefore, they may have a positive impact on adsorption of CO, which is a reactant.
There have been many studies done on the FT synthesis for a long time, from almost 90 years in the past until now. Interestingly, because variations can occur during catalytic reactions where partial catalysts are stabilized, a controversy has flared up over the active species of catalysts involved in the actual FT synthesis, whether the active species of the catalysts is a metallic iron surface, or surface of iron carbide or bulk iron carbide, or iron oxide.
In this regard, most research findings report that iron carbide particles have a crucial impact on the reaction. A particular finding states that it is important to properly form a Hägg carbide (χ-Fe5C2), which is the most active species in the FT synthesis among many iron carbides present as various phases including Hägg carbide (χ-Fe5C2), pseudo-hexagonal iron carbide ({acute over (ε)}-Fe2.2C), hexagonal iron carbide (ε-Fe3C), Eckstrom-Adcock iron carbide (Fe7C3), cementite (θ-Fe3C), etc. (Weckhuysen, B. M. et al. Chem. Soc. Rev., 2008, 37, 2758).
However, it is well known in the art that it is very difficult to obtain Fe5C2 particles in a pure state, and accordingly, the reactivity thereof in the FT synthesis has not well been reported.
Meanwhile, the method for preparing iron catalysts used in the Fischer-Tropsch synthesis primarily includes a co-precipitation method or a wetness impregnation method, and the catalysts prepared by the methods above are widely used (Korean Patent No. 10-1087165 titled “The method for preparing of Fe based catalyst used in Fischer-Tropsch synthesis reaction and that of liquid hydrocarbon using Fe based catalyst”).
Also, in the case of carbon-based iron composite catalysts that have recently been developed in addition to methods described above, they may be obtained by high temperature hydrothermal reaction or solvothermal reaction utilizing a surfactant (Zong et al. J. Am. Chem. Soc. 2010, 132, 935). However, such reactions still had difficulties in obtaining pure Hägg carbide (χ-Fe5C2) phases, which show high activity in the FT synthesis, and also had a disadvantage in scaling up due to high cost and time concerns resulting from complicated procedures required during the synthesis.